In drying a moving web of material, such as paper, film or other sheet material, it is often desirable that the web be contactlessly supported during the drying operation, in order to avoid damage to the web itself or to any ink or coating on the web surface. A conventional arrangement for contactlessly supporting and drying a moving web includes upper and lower sets of air bars extending along a substantially horizontal stretch of the web. Heated air issuing from the air bars floatingly supports the web and expedites web drying. The air bar array is typically inside a dryer housing which can be maintained at a slightly sub-atmospheric pressure by an exhaust blower that draws off the volatiles emanating from the web as a result of the drying of the ink thereon, for example.
One example of such a dryer can be found in U.S. Pat. No. 5,207,008, the disclosure of which is hereby incorporated by reference. That patent discloses an air flotation dryer with a built-in afterburner, in which a plurality of air bars are positioned above and below the traveling web for the contactless drying of the coating on the web. In particular, the air bars are in air-receiving communication with an elaborate header system, and blow air heated by the burner towards the web so as to support and dry the web as it travels through the dryer enclosure.
Regenerative thermal apparatus is generally used to incinerate contaminated process gas. To that end, a gas such as contaminated air is first passed through a hot heat-exchange bed and into a communicating high temperature oxidation (combustion) chamber, and then through a relatively cool second heat exchange bed. The apparatus includes a number of internally insulated, heat recovery columns containing heat exchange media, the columns being in communication with an internally insulated combustion chamber. Process gas is fed into the oxidizer through an inlet manifold containing a number of hydraulically or pneumatically operated flow control valves (such as poppet valves). The process gas is then directed into the heat exchange media which contains "stored" heat from the previous recovery cycle. As a result, the process gas is heated to near oxidation temperatures by the media. Oxidation is completed as the flow passes through the combustion chamber, where one or more burners are located (preferably only to provide heat for the initial start-up of the operation in order to bring the combustion chamber temperature to the appropriate predetermined operating temperature). The process gas is maintained at the operating temperature for an amount of time sufficient for completing destruction of the volatile components in the process gas. Heat released during the oxidation process acts as a fuel to reduce the required burner output. From the combustion chamber, the process gas flows through another column containing heat exchange media, thereby cooling the process gas and storing heat therefrom in the media for use in a subsequent inlet cycle when the flow control valves reverse. The resulting clean process gas is directed via an outlet valve through an outlet manifold and released to atmosphere, generally at a slightly higher temperature than inlet, or is recirculated back to the oxidizer inlet.
According to conventional combustion science, each type of burner flame (e.g., premix flame, diffusion flame, swirl flame, etc.) burns with a different optimal burner pressure ratio of fuel to combustion air, by which optimal stoichiometric low emission concentrations in the burner flue gas appear. It is therefore important to control or maintain the desired optimal burner fuel/air pressure ratios of the burner. Failure to closely regulate the burner air/fuel ratio over the range of burner output can lead to poor flame quality and stability (flameout, yellow flames, etc.) or excessive pollution (high NO.sub.x, CO).
To that end, U.S. Pat. No. 4,645,450 discloses a flow control system for controlling the flow of air and fuel to a burner. Differential pressure sensors are positioned in the air flow and gas flow conduits feeding the burner. Optimal differential pressures of the air and fuel flow are determined through experimentation and flue gas analysis and stored in a microprocessor. These optimal values are compared to measured values during operation, and the flow of air and/or fuel to the burner is regulated based upon that comparison by opening or closing respective valving. This system does not sense the back pressure on the burner. It also generates a fuel flow "signal" indicative of the rate of fuel into the burner rather than through the burner.
Mechanical valves used in conventional systems are connected by adjustable cams and linkages to control the volumetric flow rates of the air and fuel. However, if the air density changes due to atmospheric pressure and/or temperature variations, the air fuel ratio is upset. In addition, mechanical valves are subject to wear and binding of the cams and linkages over time, and considerable skill is required to adjust the device. Systems which use mass flow measuring devices are cost prohibitive.
It is therefore an object of the present invention to optimize the mix of fuel and air in a burner.
It is a further object of the present invention to provide a control system for a burner and thereby increase the efficiency of the burner.
It is another object of the present invention to reduce the flue gas emissions of a burner.